1. Field of the Invention
This invention relates to laser techniques for effecting high volume welding of workpieces and more particularly, in a preferred embodiment, to apparatus and methods for maintaining a constant optical path between a focusing lens, associated with a processing laser, and a succession of fixed workpieces, so as to produce a constant light spot size during a scan of the workpieces.
2. Description of the Prior Art
In prior art laser welding apparatus, it is well known to use light scan systems to locate parts spaced along a workpiece plane by detecting the light reflected from the parts to initiate triggering of a high-power pulse, or Q-switched high-power laser. Various optic arrangements, electrical control, and mechanical motion strategies and combinations thereof have been employed to develop efficient automatic, production line, welding systems to effect proper welds.
One method for automatic welding of parts is described in British Pat. No. 1,153,282 dated May 29, 1969. There, a system is described that scans, by a search-scan technique, a workpiece in the form of a pair of wires wherein it is assumed that the two wires touch one another at one point. An object of the search scan is to locate this point where the wires touch.
A light detection means responsive to light reflections from the wires provides a two-pulse signal indicative of light reflections from both wires, and a one-pulse signal upon the reflection of light from the point where both wires touch. Means responsive to the single pulse signal is provided for arresting the scan and initiating the welding of the workpiece at the detected point.
This method for automatic welding, however, does require the search-scan process to be interrupted in order to effect a weld. The interruption of the scan appears to be an inefficient procedure for welding a large array of workpieces spaced along the workpiece plane since considerable time could be consumed.
Another method for automatically welding of parts is described in U.S. Pat. No. 3,485,996 issued Dec. 23, 1969, to Chiou et al. There, a laser welding device is used to connect an integrated chip to a substrate. A chip lead is positioned adjacent to a substrate land and a decal having a gold strip is positioned over both the lead and the land. The decal strip has a conductive gold line which accurately overlaps, for example, one of the substrate lands and one of the chip leads.
For welding to the substrate, the assembly is passed under an optic-laser system so that the strip overlaying the substrate land may be positioned at the focal point location and automatically welded. A high intensity light is directed at the focal point location or target past which the gold strip or connector overlaying the land of the substrate is moved. The light impinges on the gold strip and reflects onto a light detecting means. The light detecting means provides a signal for triggering into emission a processing laser that is focused at the focal point location or target to effect the weld.
The moving of a series of substrates with the chip and decal attached under an optic-laser system appears to reduce the efficiency of an automatic welding operation since the chip leads must be sufficiently affixed to the substrate by the decal or other means to sustain any disruption of the connections during transit.
A further apparatus for laser welding during a scan might provide means for welding a plurality of fixed workpieces spaced along a workpiece plane. Such a system may include several workpiece planes on a rotary turntable, the turntable being intermittently indexed to step each workpiece plane under a processing laser. Such a system would require a search scan beam to be moved over the workpiece plane. Such a system might include a rotating mirror positioned between a focusing lens and the workpieces for deflecting both the scanning and processing beams over the workpiece plane.
However, the use of a rotating mirror creates a problem in that the optical paths between the lens and each workpiece would vary over the course of a scan, resulting in a different size weld at each workpiece location.
It should also be recognized that in precision welding systems, of the type wherein the tolerances of the weld zones are of the same order of magnitude as the laser spot size, the use of numerical controlled techniques, or tape controllers for indexing workpieces would be impractical. Manual repositioning would be a requirement to account for variations and tolerances of parts.